Is ESXi 5.5.0 U3b compatible with my current VCenter and host?
I'd suggest you bookmark the following links for this and future update.
According to the HCL the Gen9 hosts are supported for ESXi 5.5 U3. However, the first step is to update vCenter Server (see Interop Matrix), so that the new hosts can connect to it.
As a side note: HPE Support document - HPE Support Center may be important for you.